Insulated gate bipolar transistors (IGBTs) are well known and a typical structure and process of manufacture is described in co-pending U.S. patent application Ser. No. 08/316,112, filed on Sep. 30, 1994 (IR-1199), which is a continuation of Ser. No. 08/041,136, filed on Mar. 30, 1993 and now abandoned, which is a continuation of Ser. No. 07/521,177, filed on May 9, 1990 and now abandoned, the subject matter of which is incorporated herein by reference; and in British Patent No. 2,243,952 (IR-988). The channel length in such devices is typically that produced by a channel drive in of boron having an implant dose of 6 E15 per cm.sup.2 followed by a drive for approximately 120 minutes, and followed again by an arsenic source implanted at 3 E15per cm.sup.2 and driven in at about 975.degree. C.
This process has produced commercially acceptable IGBT devices with given forward voltage drops and switching power losses. Such devices are made by the International Rectifier Corporation of El Segundo, California.
It is always desirable to reduce the forward voltage drop of an IGBT and to decrease its switching power loss, namely to increase its; safe operating area (SOA). It is also desirable to reduce the minority carrier lifetime ("lifetime killing") to increase the switching speed of the IGBT by reducing the lifetime of the minority carriers in the silicon. Though electron irradiation may be used to reduce minority carrier lifetime, the dose that can be irradiated into the IGBT must be less than 4 megarads to avoid "switchback" and is typically less than 3 megarads.
It is therefore desirable to provide a process which permits a substantial reduction in both forward voltage drop and SOA and allows electron irradiation at higher doses without switchback.
Moreover, the effect of the irradiation can be annealed out at die bond temperatures. The device must therefore be subsequently soldered at lower temperatures to avoid this annealing, which complicates the assembly process. Therefore, doping with a heavy metal, such as platinum or gold, is preferred for lifetime killing. However, heavy metal doping increases the apparent resistivity in the active region between bases.
It is therefore also desirable to be able to use heavy metal doping in an IGBT without increasing forward voltage drop above that of a comparable electron irradiated IGBT.